I have a friend who is a team x hardcore fans and his prediction of that particular team is way better than average.
Too many factors to consider in general, but I would like to think that most fans know their team better than the overall league. However, watching every single team, is beyond achievable, as often matches clash, unless you want to dedicate your life to watching it all. Depending on the sport, that could be several hours per game.
For football/soccer I imagine a lot of people base it on the highlights of each team, with pundits pitching in. The perfect show for this in the UK, would be MOTD (Match of the day) since MOTD has highlights, as well as various pundits weighing in on their opinion of each team on that day.
However, it all depends on the team your friend supports. If they are a particularly good team in the league then statistically they are more likely to win more games, and since your friend supports the team I'd wager that he would be predicting for them to win more often than not.